1. Field of the Invention
The present invention relates to a rake receiver in a mobile communication system and a control method thereof. More particularly, the present invention relates to a rake receiver and a control method thereof for reducing power consumption of a portable terminal.
2. Description of the Related Art
A receiver in a mobile communication system performs communications in a wireless environment, and thus the communication quality thereof is liable to deteriorate in comparison to wire based communications. The most influential factor that affects transmission quality when communications are performed using electromagnetic waves in a wireless environment is fading due to multipath. Multipath Fading due to the phase differences between signals which reach the receiver through different paths, i.e., a difference in time delay. Multipath fading reduces the strength of the signal, causing transmission errors, and the time delay causes interference.
A Code Division Multiple Access (CDMA) communication system using a band spread communication technique solves problems caused by signal multipath by using a rake receiver. The rake receiver is a receiver which separates and demodulates two input signals having a time difference between them, and thus obtains time diversity by recognizing the two input signals, which have a time difference or which are different from each other, as independent signals.
The construction and operation of a conventional rake receiver will now be explained with reference to FIGS. 1 to 4.
FIG. 1 is a block diagram of a conventional rake receiver.
As described, the same signal may be transmitted through two or more transmission paths due to the wireless environment in a mobile communication system. Accordingly, as shown in FIG. 1, analog signals received through receiving paths 1001 . . . 100M (hereinafter referred to as receiving paths 100) are converted into baseband digital signals by an analog-to-digital converter (ADC) 110, and the baseband digital signals are respectively input to a searcher 120 and a controller 130. The searcher 120 checks intensity levels of the signals received through the respective paths, detects valid paths through which signals having intensities above a predetermined level are received, and informs the controller 130 and a finger unit 140 of the detection. Meanwhile, the controller 130 receives information about the valid paths and information about fingers which will be allocated with the valid paths from the searcher 120, and transmits to the finger unit 140 the signals received from the valid paths among the receiving paths 100 from the ADC 110 and an enable command signal so that the received signals can be demodulated. The finger unit 140 estimates the original signals received through the valid paths, and sends the estimated original signals to a combiner 150, and the combiner 150 combines the received original signals to estimate the original signal received through the several receiving paths.
FIG. 2 is a block diagram illustrating an inner construction of the finger unit of the conventional rake receiver.
As shown in FIG. 2, timing controllers 141 generate control signals according to the control signal from the controller 130, transmit the generated control signals to pseudo noise (PN) position calculators 142 and PN sequence generators 143, and detect time delays of the signals received from the controller 130. The PN sequence generators 143 are initialized simultaneously with the searcher 120 by initialization signals input from the timing controllers 141. The initialized PN sequence generators 143 of respective fingers 140-1 to 140-F move sequences to positions of allocated PN sequences of demodulation paths, which are designated by the searchers, through slewing, and restore the spread signals to their despread state using the time delay information detected by the timing controller.
Hereinafter, the operation of the rake receiver as constructed above will be explained with reference to FIGS. 1 to 3. FIG. 3 is a flowchart illustrating the operation of the conventional rake receiver.
If the searcher 120 and the finger unit 140 are enabled at step 200, the searcher 120 and the PN sequence generators 143 of the finger unit 140 are simultaneously initialized at step 210. In this case, the generation of the PN sequences is implemented by shift registers, and the initialization means that the searcher 120 and the PN sequence generators 143 of the respective fingers initialize the shift registers to their initial PN sequence states at an appointed time. The PN positions at this time are set to ‘0’. For example, in the case of 3rd Generation Partnership Project 2 (3GPP2), the PN positions refer to positions where ‘1’ appears after 14 ‘0’ sequences.
The searcher 120 searches the PN positions of the valid paths based on the highest energy level detected at step 220. The searcher 120 allocates the PN positions of the valid paths obtained at step 220 to fingers selected among a plurality of fingers 140 at step 230, and releases other fingers.
However, since it is essential to reduce the power consumption in small-sized appliances such as portable terminals, the power supply to the fingers, which have been released and are temporarily not in use according to the change in environment, should be intercepted, or the system clock should be completely gate-off.
Steps 240 to 280 refer to a process of performing a gate-off of idle fingers through the controller 130. The controller 130 sets the finger having the number f (i.e., finger #f) to ‘1’ at step 240. The controller 130 determines whether the finger #f exists in hardware at step 250. If the finger #f exists in hardware as a result of the determination at step 250, the controller 130 determines whether the finger #f is allocated to the PN position of the valid path at step 260. If the finger #f is not allocated to the PN position of the valid path as a result of the determination at step 260, the controller 130 disables functions of the finger #f except for the PN sequence generator 143 at step 270. If the finger #f is allocated to the PN position of the valid path as a result of the determination at step 260, the functions of the finger should be kept in an enable state, and thus the controller 130 increases the finger number by ‘1’ at step 280, and returns to step 250. If the steps 250 to 280 are performed as many times as the number of fingers which exist in the finger unit, the controller returns to step 220.
In the PN position as described above, the finger achieves fine timing in a chip using an internal time tracker, and if the designated path moves according to time, it tracks the path. The PN sequences are updated at predetermined rates, and the phases of the sequences are adjusted by adjusting update rates of shift registers according to a command for slewing to the allocated position according to the operation of the searcher 120. FIG. 4 illustrates the phase changes of the sequences according to the update-rate adjustment performed by the respective fingers.
In FIG. 4, fingers 1 to 4 are fingers the PN positions of which are set to ‘0’ at a time t0 as described in step 210. If there is no PN position movement according to the slewing command or the operational result of the timing controller 141, the PN sequences are updated at the predetermined rates, and at this time, there is no change of the PN position. That is, referring to the finger 2, the PN sequences 0, 1 and 2 are updated at the predetermined rates, and the corresponding PN positions are set to ‘0’, causing no change. However, if the update rate of the PN sequence is adjusted and the PN sequence 3 is not updated to the PN sequence 4 at a time t1, the PN position is corrected to ‘1’, and is shifted as much as the time error first measured by the time tracker. In this case, the PN position is moved in the unit of a ½ chip until the PN sequence is updated to the PN sequence 4 at a time t2.
In contrast, the finger 4′ in FIG. 4 is a finger to which the PN position is not allocated at step 230, and all functions of the finger 4′ including the PN sequence generator 143 are disabled at step 270. That is, the finger 4′ has not been initialized with the searcher 120 at the time t0, and thus has not generated the PN sequence. However, the PN position of the finger 4′ cannot be known at a time t3 when the PN position is allocated to the finger 4′ due to the increase of the valid paths allocated to the finger. In other words, an error occurs in symbol demodulation when the power supply to the respective finger is intercepted or the applied system clock is completely gate-off, and a new path is allocated thereto by the searcher, and this affects the performance of the receiver. In order to overcome this, the PN sequence generators 143 and control units of the respective fingers, which may be used at any time, should always be in a power supply state and in an enable state, respectively.